This invention relates generally to a composition for forming transparent, electrically conductive coatings. More particularly, the present invention relates to substantially stable dispersions of nanocrystalline materials that form transparent conductive coatings.
Transparent conductive coatings are generally characterized by low electrical resistance, a high transmittance of visible light, and good film strength. Such coatings may function to dissipate static charge, reduce electromagnetic radiation, or absorb and/or reflect specific types of radiation. Accordingly, such coatings are used in a wide range of devices, including window materials for solar cells, transparent electrodes, liquid crystal displays, reflective layers in architectural glasses, and microelectronic conductive components.
As defined in terms of sheet resistance by the U.S. Department of Defense, xe2x80x9cconductivexe2x80x9d is less than 105 xcexa9/(ohms per square), xe2x80x9cstatic dissipativexe2x80x9d is 105-109 xcexa9/, and xe2x80x9canti-staticxe2x80x9d is 109-1014 xcexa9/.
Transparent conductive films are commonly made from an oxide semiconductor of which indium-tin oxide (xe2x80x9cITOxe2x80x9d), which is an indium oxide containing a minor amount of tin oxide, is typical. In the case of conductive applications that do not require transparency, suitable electronic conductors include carbon fibers, metal fibers, metal-coated fibers, and aluminum flakes.
Two of the most common materials employed in static dissipative applications are carbon black and doped metal oxides. Sub-micron-sized antimony tin oxide (ATO) is a static dissipative material available as ATO primary particles or ATO doped SiO2, TiO2, or mica.
Anti-static materials are typically hygroscopic and function as xe2x80x9cionic conductorsxe2x80x9d by trapping a thin layer of moisture, which help prevent the accumulation of static charge. Such materials include compounds such as quaternary ammonium salts, sulfonates, and fatty acid esters.
Three known processes apply conductive films to substrates, namely (1) a process of sputtering film precursors, (2) a process of chemical vapor deposition (xe2x80x9cCVDxe2x80x9d) of film precursors, and (3) a process of the applying film precursors from dispersion. In the case of sputtering, the substrate is masked, placed in a vacuum chamber, and the film applied during sputtering. CVD processes are similar to sputtering. In the case of dispersion, the techniques of spin coating, dip coating, or spraying may be employed to apply the dispersion to the substrate. In order to prepare film precursors for dispersion applications, sol-gel chemistry and mechanical attrition are typically employed. Sol-gel materials are organic solvent-based dispersions.
Of the processes identified above, both the sputtering and CVD processes, which require the use of complicated equipment, suffer from the disadvantages associated with high start-up and maintenance costs. Accordingly, the dispersion process of applying the film precursor is the generally preferred process of applying a film precursor.
Processes employing sol-gel dispersions are problematic, however, in that such dispersions are unstable due to ongoing chemical interactions between particles or sol-gel precursors. Consequently, large particulates or aggregates form from the dispersion, thereby yielding films of poor optical quality. Dispersion instability leads to relatively short operational lifetimes (shelf-life). For example, conventional sol-gel derived dispersions must be shipped frozen or refrigerated using dry ice, other suitable refrigerant, or using some other suitable refrigeration method, in order to reduce the continued reactivity and chemical interactions among the particles forming the dispersion, as described above. Moreover, most dispersions are formulated by adding a complex mixture of principally organic solvents. The formulations have short shelf lives, contain large conductive particles (which negatively affect optical quality), or require high cure temperatures that limit their application.
The coating composition disclosed herein forms transparent, electrically conductive coatings from nanoparticles. The preparation process disclosed herein provides a substantially stable composition suitable for use in forming transparent, electrically conductive films.
In one aspect of the present invention, a process produces a substantially stable aqueous dispersion of nanocrystalline particles for forming a transparent conductive coating. The process comprises the steps of:
(a) adding a nanocrystalline material to water, the nanocrystalline material comprising primary particles of metal or metal oxide having a substantially spherical shape; and
(b) mixing the nanocrystalline material and water to form an aqueous dispersion.
In another aspect of the present invention, a substantially stable aqueous dispersion of nanocrystalline particles, which forms a transparent conductive coating, is prepared by the process identified above.
In yet another aspect of the present invention, a process is provided for applying a substantially transparent conductive film. The process comprises the steps of
(a) adding a nanocrystalline material to water, the nanocrystalline material comprising primary particles of metal or metal oxide having a substantially spherical shape;
(b) mixing the nanocrystalline material and water to form an aqueous dispersion;
(c) adding a film forming agent to the aqueous dispersion;
(d) adding a diluent to the aqueous dispersion to make a formulation; and
(e) applying the formulation to a substrate.